In the technical field of sealing devices, the prior art teaches the use of ultrasonic sealing devices, as this type of technology allows a precise and “cold” processing of the materials, such as, for example, the sealing of edges on thermoplastic films.
An ultrasonic sealing device comprises:                a generator of electrical power signals with frequencies generally between 20 and 50 kHz;        a converter which converts the electrical power signals into mechanical vibrations of equal frequency and defined amplitude;        an amplifier (optional) or booster, which increases the amplitude of the mechanical vibrations,        a sonotrode which transmits the mechanical vibrations to the material to be sealed; and        a anvil which operates as a contact element for the material to be sealed and the sonotrode.        
In practice, the heat sonotrode transmits energy by contact to the material to be sealed, which, by melting, is sealed.
There are two prior art types of sealing devices: the so-called “heat-sinking” devices, in which the sonotrode is movable in an alternating fashion along a single direction towards, and away from, the material to be sealed, and the anvil is stationary, or rotary; and the rotary sealing devices in which both the sonotrode and the anvil rotate, generally continuously about mutually parallel axes of rotation.
An example of sealing devices of the heat-sinking type is shown in patent WO 2004/091894.
An example of sealing devices of the rotary type is shown in patent US 2012/0012258.
In the heat-sinking sealing devices, the material to be sealed is generally moved intermittently, that is, it is positioned stationary between anvil and sonotrode during the sealing, and away from anvil and sonotrode once the sealing has been completed. The productivity of these sealing devices is generally limited.
Alternatively, the material to be sealed can also be moved continuously, thus increasing the productivity, but the high level of friction between the material and the sonotrode and the anvil has an adverse affect on the quality of the sealing, creating creases on the material and dissipating considerable quantities of energy.
The rotary sealing devices overcome the problems of friction of the heat-sinking sealers, but they have further drawbacks of an energy nature, because only a minimum part of the energy supplied to the sonotrode is actually used for sealing.
In effect, in the rotary sealing devices, the sonotrode vibrates axially along the respective axis of rotation and radially along radial directions perpendicular to the axis of rotation.
Only the radial vibrations, which correspond to approximately a quarter of the total energy supplied to the sonotrode, are used for sealing. The axial vibrations of the sonotrode are not used and, consequently, the corresponding energy (equal to approximately three-quarters of the total energy supplied to the sonotrode) is lost.